Motor Starting Current Calculator Formula

Understand the math behind the motor starting current calculator. Each variable explained with a worked example.

Use the Motor Starting Current Calculator

Formulas Used

Full Load Current (FLA)

full_load_amps = fla

Starting (Inrush) Current

starting_current = fla * starting_multiplier

Starting kVA Demand

starting_kva = fla * starting_multiplier * sqrt(3) * voltage / 1000

Input Power

input_kw = watts / (efficiency_pct / 100) / 1000

Variables

VariableDescriptionDefault
motor_hpMotor Horsepower(HP)10
voltageOperating Voltage(V)480
efficiency_pctMotor Efficiency(%)90
power_factorPower Factor at Full Load0.85
starting_multiplierStarting Current Multiplier6
wattsDerived value= motor_hp * 746calculated
flaDerived value= watts / (sqrt(3) * voltage * efficiency_pct / 100 * power_factor)calculated

How It Works

Motor Starting Current

Electric motors draw significantly more current during startup than at full speed. This inrush can be 6-8 times the full-load current.

Formulas

FLA (3-phase) = HP x 746 / (sqrt(3) x V x Eff x PF)

Starting Current = FLA x Multiplier

Starting Methods and Multipliers

  • Direct-On-Line (DOL): 6-8x FLA
  • Star-Delta: 2-3x (reduced torque)
  • Soft Starter: 2-4x (adjustable)
  • VFD: 1-1.5x (best control)

    • Worked Example

    10 HP motor, 480 V 3-phase, 90% efficient, PF 0.85, DOL start (6x).

    motor_hp = 10voltage = 480efficiency_pct = 90power_factor = 0.85starting_multiplier = 6
    1. 01Motor watts: 10 x 746 = 7,460 W
    2. 02FLA: 7460 / (1.732 x 480 x 0.90 x 0.85) = 7460 / 635.6 = 11.7 A
    3. 03Starting current: 11.7 x 6 = 70.4 A
    4. 04Starting kVA: 70.4 x 1.732 x 480 / 1000 = 58.5 kVA

    Frequently Asked Questions

    Why is starting current so high?

    At standstill, the motor's back-EMF is zero, so impedance is very low. As the motor accelerates, back-EMF rises and current drops to normal.

    How does inrush affect the electrical system?

    Large inrush can cause voltage dips, nuisance breaker trips, and flicker. Size feeders and breakers for the starting current.

    When should I use a soft starter or VFD?

    For motors above 10 HP on weak supplies, frequent starts, or when gradual acceleration is beneficial for the driven equipment.

    Ready to run the numbers?

    Open Motor Starting Current Calculator